1. Field Of The Invention
This invention relates to a strain relief device and, more particularly, to a strain relief device for use in mounting an electronic component to the surface of a printed circuit board.
In the field of electronic circuitry and packaging, there has been a need to develop ever more densely packed circuitry for use in electronic components and devices, such as those used in the computer industry for example. Semiconductor devices in particular have been the subject of much innovation in the field of integration. Accepting the need to place an increasing number of circuits in a given space, one of the best solutions in providing such sophisticated electronic circuitry in a relatively small package has been to associate one or more complex integrated circuit chips in conjunction with, or in proximity to, a printed circuit board.
It has been found that bonding chips directly to boards by means of solder, for example, is often unsatisfactory. In certain operating situations, it is the bond that fails before its associated chip or board. Accordingly, one solution is to mount integrated circuit chips to a substrate which, in turn, is mounted to the surface of a printed circuit board.
A vehicle for accomplishing this goal has incorporated metallized ceramic substrates, as taught in U.S. Pat. No. 3,921,285, issued to Krall and assigned to the present assignee, which comprise a rigid planar ceramic chip carrier to which is attached a rigid silicon integrated circuit chip.
In order to utilize both sides of a printed circuit board and to economize the size of printed circuit board surfaces and in order to decrease electrical and mechanical problems associated with pin in holes--so called thru holes--in certain circumstances, mounting components to the surfaces of boards has been found to be advantageous.
Dual in-line packages (DIPs) are conventionally mounted to the surface of printed circuit boards. Generally these devices are rectangular and have one row of electrically conductive leads attached to each of the two longest edges. The amount of circuitry and the complexity thereof are, of course, limited to that for which two parallel rows of leads are adequate.
Devices that are mounted with connections around the periphery of course permit more connections than do DIPs. Similarly, more sophisticated electronic components, such as highly integrated circuit chips having connections in a two dimensional array pattern, have the advantage of permitting more circuitry connections to be made in the same space than even peripherally leaded structures.
The leads or pins must perform two functions: provide a mechanical, physical link between the chip, through the ceramic substrate, to the board; and provide an electrical connection so that circuitry on the chip can communicate with electrically conductive paths on the surface of or within the board.
A problem arises when array substrates are mounted directly to printed circuit boards. The substrates themselves tend to be relatively rigid and inflexible, but the thermal coefficients of expansion differ between the material of which a printed circuit board is comprised and a second material of which a substrate to be mounted is comprised, causing a strain therebetween. As an example when subjected to changes of temperature, an epoxy printed circuit board tends to expand at as much as five times the rate of ceramic that is used to form a substrate. Moreover, flexure of the printed circuit board and other stresses such as mechanical vibration, shock and torque can also occur, giving rise to strain.
Due to the nature of the aforementioned stresses that are likely to occur over the lifetime of an electronic product, movement of a substrate relative to the board on which it is mounted may occur in the X and Y directions (i.e., along the plane of the printed circuit board) and/or in the Z direction (i.e., perpendicular to that plane). Such movement could not be tolerated in a surface mounting technique without an appropriate strain relief mechanism.
Shape of a strain relief mechanism is crucial in obtaining relative movement of two ends of the structure without exceeding the modulus of elasticity. For example, the helical structure of an automobile road spring accomplishes such a shock absorbing function in a relatively small amount of space. In the present electronic packaging case, a pin of sufficient length to provide strain relief can be folded, in accordance with the present invention hereinafter further described, in order to reduce vulnerability of short circuiting and to provide movement along the Z-axis (i.e., to provide vertical compliance within a reasonable length). But helical winding is difficult to implement and has electrical disadvantages.
The dimensional limitation alluded to above--height above the surface of a printed circuit board--often occurs due to other packaging requirements that facilitate loading a so-called populated board into a rack or cage. The space between one populated board and another or between a populated board and a cage wall may dictate limits to the board thickness. Thus, what is needed is a surface mounting technique or device that allows for strain caused by flexure, torque, thermal expansion and the like but does not require a great amount of space.
An article titled "Directly Attached Integrated Circuit Lead Frame" by W. R. DeBoskey, IBM Technical Disclosure Bulletin, Vol. 15, No. 1, page 307 (June, 1972) discloses a lead frame directly attached to integrated circuit chips by solder-reflow joints. Such a mechanism minimizes or eliminates the effect of thermal mismatch between a chip and a lead frame. In the disclosed apparatus, however, the chip is secured to the frame housing so that, while thermal effects are neutralized and an electrical connection is established, no mechanical connection is made.
An article titled "Cooling System For Semiconductor Modules" by H. Andres et al, IBM Technical Disclosure Bulletin, Vol. 26, No. 3B, page 1548 (August, 1983) discloses a ceramic substrate to which semiconductor chips are soldered by solder balls. Resilient pins or wires are used to connect the solder balls of chips to pads on a surface of a substrate. Once again, the chips themselves are connected to a frame structure. No mechanical connection is made by means of the resilient pins.
An article titled "Solder-Filled Elastomeric Spacer" by K. Hinrichsmeyer et al, IBM Technical Disclosure Bulletin, Vol. 27, No. 8, page 4855 (January, 1985) discloses a solder-filled elastomeric spacer which permits soldering semiconductor chips to substrates having a thermal coefficient of expansion different than that of the semiconductor material. Solder strings accommodate mechanical tensions resulting from the different thermal coefficients of thermal expansion. It is the ductility of solder--a plastic deformation phenomenon--that provides tension relief to this structure.
An article titled "Pin Structure For Heatless Replacement of Pin-Type Modules" by L. Baffaro, IBM Technical Disclosure Bulletin, Vol. 21, No. 9, page 3724 (February, 1979) discloses a pin structure for use with modules and printed circuit boards that permits heatless replacement of modules for repair or engineering changes. The purpose of this structure is to provide a simplified method of replacing a chip. The shape of the connector pins actually provides a redundant conductor for purposes of re-work. The pin is shaped so as to return upon itself above the upper surface of the chip so that it may be cut and replaced when desired without heating or removing pins from a printed circuit board.
U.S. Pat. No. 4,396,935 issued to Schuck discloses an integrated circuit package for flat circuit elements such as chips and an electrical connector for receiving such a package. The electrical connector is a cylindrically shaped hollow socket. The inner cylindrical wall contains resilient pin-like connections arranged in a circle for making contact with the corresponding conductor of the integrated circuit package. Such a configuration of pins is normally used to form wiping contacts, wherein one electrical contact or group of contacts brushes or wipes against a second contact or group of contacts. Fixed connections are not normally associated with such a connector apparatus.
It would be advantageous to provide an electrical assembly for use in mounting an integrated circuit chip by means of a substrate to a printed circuit board.
It would further be advantageous to provide an electrical assembly for mounting a two dimensional, matrix or array substrate to the surface of a printed circuit board.
It would also be advantageous to provide an electrical and also a mechanical link between a substrate and a board, the mechanical link being adapted to withstand strain.
It would further be advantageous to provide a strain relief mechanism so that a substrate under force that is undergoing movement relative to the surface of the printed circuit board on which it is mounted will continue to maintain an electrical and mechanical connection therewith.
It would also be advantageous to provide an electrical assembly comprising a plurality of electrical conductors which form a connection between a chip and the surface of a printed circuit board.
It would further be advantageous to provide a strain relief mechanism limited in height in order to accommodate electrical, mechanical and dimensional requirements.